Modulator driver circuit with selectable on-chip termination

ABSTRACT

A method and apparatus to accommodate differing output loads without sacrificing impedance matching in an optical modulator driver.

BACKGROUND

A high speed optical communication system may use various methods tomodulate an optical signal with data information for propagation alongan optical transmission medium such as optical fiber. Optical modulatorsmay use a driver circuit to provide an electrical signal correspondingto the data to be modulated. Impedance matching, where the maximumtransfer of power from the driver circuit to the modulator, however,takes place when the source and load impedances (in this case driver andmodulator impedances) are complex conjugates. Typically, drivers onseparate chips have been employed to drive different output loads. Thesedriver circuits, however, must support various output load valueswithout sacrificing associated impedance matching. Consequently, theremay be a need for improvements in impedance matching for opticalmodulator driver circuits while obviating the need for separatemodulator chips and chipsets to drive various output loads.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as embodiments is particularly pointed outand distinctly claimed in the concluding portion of the specification.The embodiments, however, both as to organization and method ofoperation, together with objects, features, and advantages thereof, maybest be understood by reference to the following detailed descriptionwhen read with the accompanying drawings in which:

FIG. 1 is a block diagram of a transceiver 110 utilized in high speedoptical communication systems suitable for practicing one embodiment;

FIGS. 2A-2C schematically illustrate various driver circuits inaccordance with one embodiment;

FIGS. 3A-3C schematically illustrate various driver circuits inaccordance with one embodiment;

FIG. 4A illustrates a circuit for a 50 .OMEGA. load dual driver circuitin accordance with one embodiment; and

FIG. 4B illustrates a circuit for a 25 .OMEGA. load dual driver circuitin accordance with one embodiment.

DETAILED DESCRIPTION

The embodiments relate to a modulator driver circuit having selectableon-chip back termination to accommodate various load values withoutsacrificing impedance matching between the driver and the modulationtechnique employed. Standard impedance values are associated withvarious components and devices included in optical transmissionequipment. Typical optical modulator drivers have impedance values thatmatch the impedance value of the modulator.

It is worthy to note that any reference in the specification to “oneembodiment” or “an embodiment” means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment. The appearances of the phrase“in one embodiment” in various places in the specification are notnecessarily all referring to the same embodiment.

Numerous specific details may be set forth herein to provide a thoroughunderstanding of the embodiments. It will be understood by those skilledin the art, however, that the embodiments may be practiced without thesespecific details. In other instances, well-known methods, procedures,components and circuits have not been described in detail so as not toobscure the embodiments. It can be appreciated that the specificstructural and functional details disclosed herein may be representativeand do not necessarily limit the scope of the embodiment.

Referring now in detail to the drawings wherein like parts aredesignated by like reference numerals throughout, FIG. 1 is a blockdiagram of transceiver 110 utilized in high speed optical communicationsystems suitable for practicing one embodiment. Transceiver module 110is operatively responsive to transmission medium 120 configured to allowthe propagation of a plurality of information signals. The expression“information signals,” as used herein, refers to an optical orelectrical signal which has been coded with information. An opticalcommunication is configured with transceivers at both ends oftransmission medium 120 to accommodate bidirectional communicationwithin a single line card. Additional amplifiers 130 may also bedisposed along transmission medium 120 depending on the desiredtransmission distances and associated span losses in order to provide aninformation signal having a power level sufficient for detection andprocessing by the receive functionality (not shown) of transceiver 110.

The information signals transmitted by transceiver 110 may be modulatedusing various techniques including return to zero (RZ) where the signalreturns to a logic 0 before the next successive date bit and/ornon-return to zero (NRZ) format where the signal does not return to alogic 0 before the next successive data bit. Transceiver 110 maycomprise a light source 150, such as a semiconductor laser, modulator160, modulator driver 170 and re-timer circuit or encoder circuit 180 totransmit optical signals. Re-timer circuit 180 receives informationsignals in electrical form and supplies these signals to modulator 160which provides current variations proportional to the receivedinformation signals to modulator 160. Light source 150 generates opticalsignals proportional to the received current levels for propagation overtransmission medium 120.

Light source 150 may be directly modulated obviating the need formodulator 160. In a directly modulated laser (DML) configuration, aminimum current signal, also known as a threshold current, is applied tothe laser causing the laser to operate in the lasing mode. Thisthreshold current is temperature dependant and may vary over theoperating range of the laser. In order to modulate the laser, thecurrent signal is varied between a point near the threshold currentcorresponding to an “off” state and above the threshold current tocorrespond to an “on” state consistent with the data to be modulated.This technique is used so that the laser remains in the lasing modewhich avoids going from a true off state, below the lasing threshold, tothe lasing threshold.

In high gigabit data transmission, however, it is more difficult toswitch the laser between these two levels. Therefore, externalmodulation may be more desirable. In external modulation, a constantlight source or laser is used and the data is modulated externally usingmodulator 160. In this manner, laser 160 supplies a carrier wave at aconstant output power and is coupled to a discrete optical modulator. Aradio frequency (RF) signal supplied to the modulator from driver 170encodes the data onto the constant light source. In this manner, laser150 remains in its lasing mode and the signal is modulated externallyfrom the light source.

In one embodiment, there are two types of external modulators, namely alithium niobate (LiNbO.sub.3) Mach-Zender interferometer and anelectro-absorption (EA) modulator. EA modulators make use of eitherPockels effect or the quantum confinement Stark effect of a quantum wellwhere the refractive index of the semiconductor material is changed uponapplication of an applied voltage. EA modulators are fabricated on asingle chip with a distributed feedback (DFB) laser and may be driven atrelatively low voltage levels. Similarly, in a Mach-Zender modulator anRF signal changes the refractive index around a pair of waveguides. Themodulator has two waveguides and the incoming light is supplied to eachwaveguide where a voltage may be applied to one or both of thewaveguides. This electric field changes the refractive index so that thelight emerging from one waveguide will be out of phase with the lightoutput from the other waveguide. When the light is recombined, itinterferes destructively, effectively switching the light off. Withoutan applied field the light is in phase and remains “on” therebyproducing a corresponding modulated signal.

In either modulation technique, modulator driver 170 provides an RFsignal corresponding to the data to be modulated. As described brieflyabove, devices are designed for operating efficiencies such that theirinput and output impedances match. For example, modulator 160 may havean input impedance of 25 .OMEGA. or 50 .OMEGA.. Likewise, driver 170must have a matching impedance to avoid costly customized system orcomponent designs. Modulator driver 170 also includes a preamplifierthat converts low-level electrical data to signals having sufficientpower levels to drive optical modulator 160 without adding signaldistortion.

FIGS. 2A-2C schematically illustrate various driver circuits inaccordance with an embodiment with separate output pads therebyalleviating the need for separate ships to drive different output loads.

FIG. 2A illustrates a schematic diagram of driver circuit 170 configuredto accommodate either a 25 .OMEGA. or 50 .OMEGA. output load whilemaintaining acceptable impedance matching. Driver 170 is configured astwo essentially identical driver circuits 210 and 220 with separate backtermination or output pads Vop1, Von1 and Von2 and Vop2 which allow thedrivers to be connected or disconnected depending on the desired outputload. The circuits are essentially identical in terms of parasiticcapacitances and resistances, share power supply voltage Vdd, anddesigned to drive a 50 .OMEGA. load independently. Output pads Vop1 andVon1 are connected or wire bonded to circuit 210 and output pads Von2and Vop2 are connected to circuit 220. Driver circuit 210 includesdifferential transistor pair T1 and T2 whose source terminals areconnected to current source I.sub.1. The drain terminals of transistorsT1 and T2 are connected to output termination resistors R1 (50 .OMEGA.)and R2 (50 .OMEGA.) respectively. Input signals Vip and Vin are appliedto the gate terminals of T1 and T2 to turn the transistors on and offand likewise the constant current source I1 of circuit 210. Drivercircuit 220 is a mirror image of circuit 210, but for the individualoutput pads, and includes differential transistor pair T1′ and T2′ whosesource terminals are connected to current source I.sub.1′. The drainterminals of transistors T1′ and T2′ are connected to output terminationresistors R1′ (50 .OMEGA.) and R2′ (50 .OMEGA.) respectively. Inputsignals Vip and Vin are applied to the gate terminals of T1′ and T2′.Input signals Vip and Vin are applied to the gate terminals of T1′ andT2′ to turn the transistors on and off and likewise the constant currentsource I1′ of circuit 220.

FIG. 2B illustrates the disconnect configuration of the two drivercircuits 210 and 220 to drive a 50 .OMEGA. load. Circuit 230 representsan off chip EA modulator with R=50 .OMEGA. and an off-chip input pad235. Resistor 240 (50 .OMEGA.) and input pad 245 represents an off-chipequivalent 50 .OMEGA. termination resistance. Output pad Vop1 isconnected to input pad 235 and output pad Von1 is connected to input pad245. Output pads Von2 and Vop2 are left open and current source I.sub.1′of driver circuit 220 is disabled thereby saving half the circuit power.In this manner a selectable on-chip back termination to accommodate a 50.OMEGA. load value is achieved essentially using the dual driver circuit210.

FIG. 2C illustrates the connection configuration of the two drivercircuits 210 and 220 to provide a 25 .OMEGA. termination. Similar tocircuit 230, circuit 250 of FIG. 2C represents an off chip EA modulatorwith R=50 .OMEGA. and an input pad 255. Resistor 260 and input pad 265represent an equivalent termination resistance (25 .OMEGA.). Input pad255 is connected to output pads Vop1 and Vop2. Input pad 265 isconnected to output pads Von1 and Von2. Since the termination resistorsare in parallel a 25 .OMEGA. driver is provided using the dual drivercircuits 210 and 220.

FIGS. 3A-3C schematically illustrate various driver circuits inaccordance with an embodiment with common on-chip back termination oroutput pads and separate power supply terminals.

FIG. 3A illustrates a schematic diagram of driver circuit 170 configuredto accommodate either a 25 .OMEGA. or 50 .OMEGA. output load whilemaintaining acceptable impedance matching. Similar to FIGS. 2A-2C,driver 170 is configured as two essentially identical driver circuits310 and 320 with common on-chip back termination or output pads Vop andVon and separate power supply terminals Vdd for circuit 310 and powersupply terminals 321 and 322 for circuit 320. By utilizing separatepower supply connections, driver circuit 320 may be connected ordisconnected depending on the desired output load. Other than the Vddconnections, circuits 310 and 320 are essentially identical in terms ofparasitic capacitances and resistances and designed to drive a 50.OMEGA. load independently. Driver circuit 310 includes differentialtransistor pair T1 and T2 whose source terminals are connected tocurrent source 11. The drain terminals of transistors T1 and T2 areconnected to output termination resistors R1 (50 .OMEGA.) and R2 (50.OMEGA.) respectively. Input signals Vip and Vin are applied to the gateterminals of T1 and T2. Driver circuit 320 is a mirror image of circuit310, but for the separate power supply connections 321 and 322, andincludes differential transistor pair T1′ and T2′ whose source terminalsare connected to current source I.sub.1′. The drain terminals oftransistors T1′ and T2′ are connected to output termination resistorsR1′ (50 .OMEGA.) and R2′ (50 .OMEGA.) respectively. Input signals Vipand Vin are applied to the gate terminals of T1′ and T2′. In thismanner, output pads Vop and Von are shared on-chip by both drivercircuits 310 and 320.

FIG. 3B illustrates the power supply disconnect configuration of drivercircuit 320 to drive a 50 .OMEGA. off-chip load. Circuit 330 representsan off chip EA modulator with R=50 .OMEGA. and an off-chip input pad335. Resistor 340 (50 .OMEGA.) and input pad 345 represents an off-chipequivalent 50 .OMEGA. termination resistance. Shared output pad Vop isconnected to input pad 335 and output pad Von is connected to input pad345. The power to circuit 320 is turned off, thereby disabling currentsource I.sub.1′ of circuit 320. In this manner, a selectable on-chipback termination is used to provide a 50 .OMEGA. terminationconfiguration essentially utilizing driver circuit 310.

FIG. 3C illustrates the connection configuration of the two drivercircuits 310 and 320 to provide a 25 .OMEGA. termination. Similar tocircuit 330, circuit 350 of FIG. 3C represents an off chip EA modulatorwith R=25 .OMEGA. and an input pad 355. Resistor 360 and input pad 365represents an equivalent termination resistance (25 .OMEGA.). Input pad355 is connected to output pad Vop and input pad 365 is connected tooutput pads Von. Power supply pads 321 and 322 are each connected to Vddproviding power to circuit 320 and driving current source I.sub.1′ ofcircuit 320. Similar to FIG. 2C, the termination resistors of FIG. 3Care now in parallel essentially producing a 25 .OMEGA. driver using thedual driver circuits 310 and 320.

FIGS. 4A and 4 b illustrate an equivalent circuit for the 50 .OMEGA. and25 .OMEGA. dual driver approach. In particular, FIG. 4A is an equivalentcircuit representing the 50 .OMEGA. load illustrated in the dual drivercircuit configuration of FIGS. 2B and 3B. Similarly, FIG. 4B is anequivalent circuit representing the 25 .OMEGA. load illustrated in thedual driver circuit configuration of FIGS. 2C and 3C.

An embodiment provides a driver that accommodates different output loadswhile maintaining impedance matching and avoiding additional chip countsassociated with individual drivers. By changing the connections betweenon-chip and off-chip output pads, as well as on chip power supplyterminals, a circuit designer can manipulate the output loads withoutsacrificing impedance matching.

While certain features of the embodiments have been illustrated asdescribed herein, many modifications, substitutions, changes andequivalents will now occur to those skilled in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the embodiments

1. A driver circuit, comprising: a first driver circuit having a firstoutput terminal operatively responsive to an output load resistancecorresponding to a modulator circuit and a second output terminaloperatively responsive to a termination resistance, said first drivercircuit having a constant current source configured to supply a currentsignal in response to an input signal applied to said first drivercircuit; and a second driver circuit having a constant current source,said second driver circuit operatively responsive to said first drivercircuit by a first and second input terminals.
 2. The driver circuit ofclaim 1, wherein said constant current source of said second drivercircuit is disabled.
 3. The driver circuit of claim 1, wherein saidfirst driver circuit further comprises a first transistor, said firsttransistor having a source terminal operatively responsive to saidconstant current source, a drain terminal operatively responsive to afirst resistor, and a gate terminal configured to receive said inputsignal.
 4. The driver circuit of claim 3, wherein said input signal is afirst input signal, said first driver circuit further comprising asecond transistor having a source terminal operatively responsive tosaid constant current source and said source terminal of said firsttransistor, a drain terminal operatively responsive to a second resistorand a gate terminal configured to receive a second input signal.
 5. Thedriver circuit of claim 4, wherein said first output terminal isconnected, at a first end, to said drain terminal of said firsttransistor and to an input of said modulator circuit at a second end,and said second output terminal is connected, at a first end, to saiddrain terminal of said second transistor and to said terminationresistance at a second end.
 6. The driver circuit of claim 1, whereinsaid second driver circuit further comprising first and second outputterminals, said output terminals being unconnected to said modulatorcircuit and said termination resistor disabling said constant currentsource associated with said second driver circuit.
 7. The driver circuitof claim 1, wherein said second driver circuit further comprising afirst transistor, said first transistor having a source terminaloperatively responsive to said constant current source, a drain terminaloperatively responsive to a first resistor, and a gate terminalconfigured to receive said input signal.
 8. The driver circuit of claim7, wherein said input signal is a first input signal, and said seconddriver circuit further comprising a second transistor having a sourceterminal operatively responsive to said constant current source of saidsecond driver circuit and said source terminal of said first transistor,a drain terminal operatively responsive to a second resistor and a gateterminal configured to receive a second input signal.
 9. The drivercircuit of claim 8, wherein said second driver circuit furthercomprising first and second output terminals, said first output terminalconnected at a first end to said drain terminal of said secondtransistor and to said termination resistance at a second end, saidsecond output terminal connected at a first end to said drain terminalof said first transistor and to an input of said modulator circuit. 10.The driver circuit of claim 1, wherein said first driver circuit furthercomprising first and second power supply terminals, said second drivercircuit further comprising first and second power supply terminals, saidfirst driver circuit further operatively responsive to said seconddriver circuit by said first and second output terminals.
 11. The drivercircuit of claim 10, wherein a voltage signal is supplied to said firstdriver circuit via said first and second power supply terminals but notsupplied to said first and second power supply terminals of said seconddriver circuit disabling said constant current source associated withsaid second driver circuit.
 12. The driver circuit of claim 1, whereinsaid first and second driver circuits are mirror images of each other.13. An optical communication system, comprising: a transmission mediumconfigured to allow propagation of optical signals; at least one opticalamplifier disposed along said transmission path for amplifying saidoptical signals; and a transceiver operatively responsive to saidtransmission medium, said transceiver comprising a light source, amodulator for modulating said information signals onto light signalsgenerated by said light source and a modulator driver for supplyingelectrical signals representing said information signals to saidmodulator, said modulator driver comprising a first driver circuithaving a first output terminal operatively responsive to an output loadresistance corresponding to said modulator circuit and a second outputterminal operatively responsive to a termination resistance, said firstdriver circuit having a constant current source configured to supply acurrent signal in response to an input signal applied to said firstdriver circuit and a second driver circuit having a constant currentsource, said second driver circuit operatively responsive to said firstdriver circuit by a first and second input terminals.
 14. The system ofclaim 13, wherein said first driver circuit further comprising first andsecond power supply terminals, said second driver circuit furthercomprising first and second power supply terminals, said first drivercircuit further operatively responsive to said second driver circuit bysaid first and second output terminals.
 15. The system of claim 14,wherein power is supplied to said first driver circuit via said firstand second power supply terminals but not supplied to said first andsecond power supply terminals of said second driver circuit disablingsaid constant current source associated with said second driver circuit.16. A method, comprising: supplying input signals to first and secondmodulator driver circuits, each of said circuits including a constantcurrent source; connecting a first output from said first modulatordriver circuit to said optical modulator; connecting a second outputfrom said first modulator driver circuit to a termination resistance;and shutting off said current source included in said second modulatordriver circuit.
 17. The method of claim 16, wherein said first andsecond modulator driver circuits are mirror images of each other. 18.The method of claim 16, further comprising: connecting a first output ofsaid second modulator driver circuit to said termination resistance; andconnecting a second output of said second modulator driver circuit tosaid optical modulator.
 19. The method of claim 16, further comprising:supplying power to said first modulator driver circuit such that aconstant current source included in said first modulator driver circuitoperates to provide a signal to said optical modulator and saidtermination resistance; and shutting off power supplied to said secondmodulator driver circuit.
 20. The method of claim 19, furthercomprising: connecting a first output of said second modulator drivercircuit to said optical modulator; and connecting a second output ofsaid second modulator driver circuit to said termination resistance.